LINQ to XML实现XML文档的增删改查

1、实例中的XML文档（Books.xml）

<?xml version="1.0" encoding="utf-8"?>
<bookstore>
  <book id="1" category="CHILDREN">
    <title>Harry Potter</title>
    <author>J K. Rowling</author>
    <publishDate>2005-08-15</publishDate>
    <price>29.99</price>
  </book>
  <book id="2" category="WEB">
    <title>Learning XML</title>
    <author>Erik T. Ray</author>
    <publishDate>2003-10-18</publishDate>
    <price>39.95</price>
  </book>
  <book id="3" category="WEB">
    <title>XQuery Kick Start</title>
    <author>James McGovern</author>
    <publishDate>2005-06-25</publishDate>
    <price>49.99</price>
  </book>
</bookstore><?xml version="1.0"